The present invention relates to a hot melt adhesive applicator and more particularly to a hot melt adhesive applicator capable of dispensing relatively large volumes of molten adhesive at relatively fast rates.
Hot melt adhesive applicators and guns are used to dispense molten adhesive. Such applicators many contain a heating chamber into which a solid cartridge of adhesive is fed and from which molten adhesive is discharged or they may be attached to a hose or pipe which supplies molten adhesive from a reservoir. A trigger operated spring-loaded needle valve usually controls the discharge of adhesive from the applicator and limits the dispensing capability to relatively small rates of flow of molten adhesive because of the narrow passage provided to the molten adhesive in such needle valve systems, such passage often being further constricted by the valve actuating means. A large pressure drop occurs in these applicators.
In recent times, large molded plastic parts such as fenders, doors, fire-walls, tailgates, hoods, and trunk lids for automobiles and trucks have been developed. The complexity of such parts makes one piece molding difficult to carry out. Consequently complete assemblies are fabricated by adhering two or more parts together. Assembly line manufacture of such parts requires intermittent, fast delivery of adhesives.
I have now invented a hot melt adhesive applicator incorporating a rotary valve, which is capable of dispensing relatively large volumes of molten adhesive at relatively fast rates. The applicator also possesses the advantage of sharp interruption in flow of molten adhesive when the valve is closed so that dripping or drooling of adhesive is prevented. The applicator comprises:
(a) a barrel mounted in a housing, the barrel having a rear section for receiving a supply of molten thermoplastic and a front section terminating in a nozzle for discharge of molten thermoplastic;
(b) heating means and temperature sensors attached to the barrel to maintain the thermoplastic in the molten state;
(c) interposed between the rear and front sections of the barrel, a valve block with a rotary valve rotably mounted in the valve block, the rotary valve containing a passage to allow flow of molten thermoplastic from the rear section to the front section of the barrel when the valve is open; and
(d) means mounted on the rotary valve to cause the valve to open and close. A seal or seals mounted on the rotary valve spindle prevent seepage of molten thermoplastic from the valve block. Advantageously, the clearance between the rotary valve and the rotary valve block is less than 0.1 mm. For hot melt adhesives applied at temperatures less than about 150.degree. C. elastomeric seals may be used advantageously. For hot melt adhesives used in structural aplications and applied at temperatures above 150.degree. C., such elastomeric seals are unsatisfactory since they tend to degrade or become embrittled and fail after a very short period of service. For applications above 150.degree. C., metal compression seals made of a suitable metal or metal alloy are more satisfactory. The seals engage in contoured fit against shoulder regions of the valve between the valve spindle and the wider portion of the valve which mates with the valve seat in the valve block.
The barrel is divided into two sections with the valve block interposed. The rear barrel section which receives molten thermoplastic adhesive and delivers it to the valve block and the front barrel section which receives molten thermoplastic adhesive from the valve block and delivers it to the applicator nozzle from which the molten adhesive is discharged, may be co-axial or may be placed at an angle to one another, with the front barrel section at an angle, to allow downward discharge of the molten adhesive. Alternatively the front barrel section may be co-axial with the rear barrel section at the receiving end adjacent to the valve block and may be inclined at the discharge end to allow downward discharge of molten adhesive. Preferably the front and rear barrel sections are co-axial to allow substantially linear flow of the adhesive in the applicator and to avoid dead spots in which thermoplastic stagnates and degrades.
The rotary valve can be a plug valve of cylindrical or truncated cone shape or a ball valve seated in a channel in the valve block adapted to receive the valve so that the surfaces of the valve and the channel are in substantially sealing contact. The valve has a suitably shaped passage bored into it, for example the passage may be circular, oblong circular or elliptical in cross-section. The passage is aligned with the discharge end of the rear barrel section and the receiving end of the front barrel section when the valve is open to allow flow of molten adhesive. For ease of assembly the valve is preferably a plug valve of cylindrical or truncated cone shape in the section which mates with the channel in the cylinder block. When the valve is a ball valve the valve block is made of two pieces which sealingly engage one another after the valve has been seated in its channel. To actuate the valve, an arm can advantageously be attached to the valve spindle, to provide a pivoting action. The arm may be moved manually, by electrical solenoid, by mechanical screw or by pneumatic or hydraulic means.
In a convenient means of manual operation, the arm is bifurcated and attached to the opposite ends of the spindle of the valve and a bifurcated pressure responsive lever acts on the two forks of the arm to open the valve when pressure is applied on the lever in opposition to springs which move the valve to the closed position when pressure on the lever is removed.
These and other features of the invention are illustrated in the drawings.